Is it possible with AT3600 to measure current and voltage (AC and DC) using an external source (power supply unit etc.)?
It is completely possible to use an external source to supply power to the part under test using the 'AC Interface'.
In principle the AT3600 will measure both ac and dc voltages using the VOC test. For example it is possible to power a power supply with a 230V MAGI test and measure 5Vdc output. If there is voltage present when the AT3600 switches relays this can cause damage, especially if there is the energy of a charged capacitor present.
In summary, an external AC source can be used with the AC interface. External voltages that are not switched by the AT3600 should not be connected because they may damage AT3600 relays.
Please also see article “AC Interface External Amplifiers and Output Transformers".